Does anyone know of a source for high temp wire in smaller quantities. I'm thinking the hall effect cable and other cables that run against the engine. I suppose they would have silicon or teflon based insulation.

not always robin   ...but certainly a protective outer insulation

there are specific insulation type eg v75 and v90   (you guessed it 70 c and 90 c temp ratings )

i havent seen any multicore + sheilded cables rated for engine work recently ...although yes they would exist ...but quite specialised
perhaps standard cable and an outer sheath (insulation )  of something high temp ?

Where do you see these charlie?

I looked at rs components and they have 120C and 280C wires but the cost. Like $800 for a 100 meter roll.

The 02 sensor wires that run to a cat in the exhaust of cars are usually shielded wires perhaps a trip to the motor wreckers would be the go

Is this the kind of thing you're looking for
Heat resistant cable

Or this
Heat resistant sleeving